Four people were wounded on Sunday when two rockets exploded in the Shiite-majority Hezbollah heartland of south Beirut, a Lebanese security source said.
"Two Grad rockets hit the southern suburbs of Beirut. One rocket struck a car showroom where four people were wounded and vehicles were damaged," the source told AFP.
It was the first time the Lebanese capital's southern suburbs have been targeted during the two-year-old conflict in neighbouring Syria, where Hezbollah has thrown its military might into the regime's fight against rebels seeking to topple President Bashar al-Assad.
Sunday's blasts came just hours after Hezbollah leader Hassan Nasrallah vowed "victory" in Syria.
"I say to all the honourable people, to the mujahedeen, to the heroes: I have always promised you a victory and now I pledge to you a new one" in Syria, he said at a ceremony marking the 13th anniversary of Israel's military withdrawal from Lebanon.
"This battle is ours... and I promise you victory," he said.
Nasrallah said Hezbollah would always stand by its ally Assad and his regime, stressing that its own interests were at stake.
Both rockets hit the Al-Shayyah area of Beirut.
The security source said the four wounded in the showroom were all Syrian workers. An AFP journalist said the second rocket hit an apartment block and caused widespread damage but no casualties.
"This incident is probably related to the conflict in Syria," the security source said.
He said the rockets were fired from Aitat in the Mount Lebanon area some 13 kilometres (8 miles) southeast of where they hit.
"The army is on the way to this area to investigate," he said.
Hezbollah's Al-Manar television channel broadcast live from Al-Shayyah in the capital, showing security forces and Interior Minister Marwan Charbel arriving at the scene of the blasts.
The fighting in Lebanon has already spilled over into the country's second city, the northern port of Tripoli, where 30 people have been killed in a week of clashes between pro-Assad Alawites and pro-rebel Sunni Muslims.
Most of the fighting in Tripoli has been between residents of the Sunni district of Bab el-Tebbaneh and Alawite residents of Jabal Mohsen.
The rebels battling the Syrian regime are mainly Sunnis. Assad belongs to the Alawite sect which is an offshoot of Shiite Islam.
"Syria is the rear guard of the resistance (Hezbollah's fight with Israel), its backbone, and the resistance cannot stay with its arms folded when its rear guard is exposed," Nasrallah said in his speech.
"We are idiots if we do not act," added the Hezbollah leader who avoids appearing in public for fear of being assassinated by Israel. His Saturday speech was broadcast by video link live on a huge screen.
The intervention of hundreds of Hezbollah fighters has given Assad the upper hand in Qusayr, a strategic central town in Syria across the border with Lebanon, that had been in rebel hands.
Syrian forces launched an assault on Qusayr last Sunday but are still meeting with fierce resistance from the rebels, as the town provides an important supply line for arms and volunteers from Lebanon.
Qusayr is a key prize for Assad's forces because of its location between Damascus and the Alawite Mediterranean coast.